Crystalline silicon-based solar cells using crystalline silicon substrates are high in photoelectric conversion efficiency, and thus have been widely and generally used in solar power generation systems. Among them, a crystalline silicon-based solar cell in which a conductive amorphous silicon-based thin-film having a band gap different from that of a single-crystal silicon is formed on a surface of the crystal silicon substrate is called a heterojunction solar cell.
Among heterojunction solar cells, a solar cell having an intrinsic amorphous silicon thin-film between a conductive amorphous silicon-based thin-film and a crystal silicon substrate is one embodiment of a crystalline silicon-based solar cell with a high conversion efficiency. By forming an intrinsic amorphous silicon thin-film between a single-crystal silicon substrate and a conductive amorphous silicon-based thin-film, generation of defect levels by formation of the conductivity-type amorphous silicon-based layer can be reduced and defects present on the surface of single-crystal silicon (principally dangling bonds of silicon) can be terminated (passivated) with hydrogen. In addition, due to a presence of an intrinsic amorphous silicon thin-film, carrier-introduction impurities can be prevented from diffusing to the surface of single-crystal silicon at the time of forming a conductive amorphous silicon-based thin-film.
As an attempt to enhance the conversion efficiency of aforementioned heterojunction solar cell, Patent Document 1 proposes that before an intrinsic amorphous silicon-based layer is formed on a crystalline silicon substrate, the crystalline silicon substrate surface is subjected to a hydrogen plasma treatment to clean the substrate surface.
Patent Document 2 proposes that a hydrogen concentration profile in an amorphous silicon-based layer is controlled by changing the hydrogen dilution ratio in multiple stages during formation of the amorphous silicon-based layer. Patent Document 2 describes that by controlling the hydrogen concentration profile, the dangling bond of silicon in the film is terminated, so that carrier recombination is suppressed, leading to enhancement of the conversion characteristics. As a technique related to a thin-film silicon-based photoelectric conversion device, Patent Document 3 proposes that formation of an amorphous silicon-based thin-film having a thickness of less than 1 nm and a hydrogen plasma treatment are repeatedly carried out. The method of repeating deposition and a hydrogen plasma treatment in this way is also referred to as “chemical annealing”. By chemical annealing, defects in the amorphous silicon-based layer are reduced.